Allow function calls in expressions and add len() built-in

- Parse function calls in parse_term() so they work inside expressions
  (e.g. z = len(x), y = len(x) + 1)
- Add len() built-in for string length in evaluator

src/vm/eval.h

@@ -168,6 +168,17 @@ size_t eval(ASTNode *node, Environment *env, void *allocator, int debug,
       printf("\n");
       return 0;
     }
+    if (strcmp(node->data.call.name, "len") == 0) {
+      if (node->data.call.arg_count == 1) {
+        size_t val = eval(node->data.call.args[0], env, allocator, debug, gc);
+        Object *obj = (Object *) JLANG_RESOLVE(allocator, val);
+        
+        if (obj->type == OBJ_STRING) {
+          return obj_new_int(allocator, obj->data.string_val.length);
+        }
+      }
+      return 0;
+    }
     printf("ERROR: funcion '%s' no definida\n", node->data.call.name);
     exit(1);
   }